Modulated power supplies as a part of envelope modulators in an enveloped tracked power supply system are well-known. FIG. 1 shows an envelope tracked power amplifier (PA) system in which the power amplifier supply voltage is modulated synchronously with the instantaneous power amplifier input power.
With reference to FIG. 1 there is illustrated a power amplifier system 40 which receives an input signal on line 30 and generates an output signal on line 32. The input signal on line 30 is provided to a IQ modulator 10, which provides an output to an optional delay adjustment block 12 in an RF input path and to an envelope detect block 18 in an envelope path.
In the RF input path the delay adjust block 12 is provided to allow for delays in the envelope path. It should be noted that in general a delay could also or alternatively be provided in the envelope path. The purpose of any delay is to align the signals in the envelope and RF paths at the power amplifier. The output of the delay adjust block 12 is connected to an upconvert block 14 which converts the input signal to an RF signal. The output of the upconvert block 14 on line 34 provides an input to the power amplifier 16, and the output of the power amplifier 16 is provided on line 32.
The envelope detect block 18 in the envelope path provides a signal which represents the envelope of the input signal to a non-linear mapping block 20, which in turn provides an output to an envelope modulator 22. The envelope modulator comprises a low-frequency path including a switcher 24 and a high-frequency path including a linear amplifier 26. The switcher 24 and the linear amplifier 26 each receive the output of the non-linear mapping block 20. The output of the switcher 24 and the output of the linear amplifier 26 are combined by a combiner 28, and the output of the combiner 28 provides a supply voltage on line 36 to the power amplifier 16.
As illustrated in FIG. 1 the envelope modulator comprises a switcher 24. The switcher 24 may comprise one or more switch mode DC-DC converters, which provide the bulk of the direct current (DC) and low-frequency (LF) output power. As also illustrated in FIG. 1 the envelope modulator comprises a linear amplifier 26. The linear amplifier 26 provides the residual high-frequency (HF) output power.
The arrangement of FIG. 1 allows the envelope modulator 22 to simultaneously achieve high efficiency and high tracking accuracy. The power amplifier supply voltage on line 36 is dynamically adjusted to be just sufficient for the power amplifier to generate the required instantaneous output power, markedly improving its efficiency.
An example use of an envelope tracked power amplifier system such as that shown in FIG. 1 is in a cellular telephone handset. Cellular telephone handsets use complex modulation with high peak-to-average power ratio. The transmit signal is typically divided into timeslots, and the average transmit power for each timeslot is set to the required level by a controller. To maximise battery life, it is important to minimise the power consumption of the power amplifier subsystem (i.e. the subsystem shown in FIG. 1) across a wide range of output powers.
A typical cellular telephone handset uses a combination of supply modulation techniques to achieve this. For example, at low output powers the power amplifier is typically powered by an average power tracking (APT) converter which provides a DC voltage which is adjusted from slot-to-slot. An APT converter may be provided by using one or more of the switchers of the envelope tracking modulator described above, by disabling the linear amplifier. At high output powers the power amplifier may instead be powered from the complete envelope tracking modulator such as that shown in FIG. 1.
The efficiency of an exemplary power amplifier including the supply converter (APT or ET) is illustrated in FIG. 2. FIG. 2 illustrates a plot of the average power amplifier efficiency (including the envelope tracking or average power tracking supply) against the average RF output power. The waveform 50 illustrates the plot for an average power tracking supply converter, and the waveform 52 illustrates the plot for an envelope tracking power converter.
As can be seen in FIG. 2, the vertical dash line 54 is indicative of a point on the average output power (of the RF output) axis at which the overall power efficiency is the same in both the envelope tracking and average power tracking modes of operation. This point at which the overall power amplifier efficiency is the same for both modes can be considered to be a “crossover” point, being the point at which the mode having the higher efficiency changes. As can be seen in FIG. 2 for average RF output powers below the dash line 54 the APT mode has a higher average power amplifier efficiency, and for average RF output power above the dash line 54 the envelope tracking mode has the higher average power amplifier efficiency.
It is an aim of the present invention to improve the efficiency of the envelope tracking solution when operating below maximum output power, thereby reducing the ET/APT efficiency crossover point.